Transconductor employing field controlled semiconductor



Sept. 30, 1952 o. M. sTUi-:TZER 2,612,567

TRANscoNDUcToR EMPLOYING FIELD coNTRoLLED sEMIcoNpUcToR Filed oct. 4, 1949 4 sheets-sheet 1 "I Ell I N VEN TOR.

l 0772714. 67.7/ E? AT? fol/Eff 4 sept. 3o, 1952 o. M. s'rUETzER 2,612,567 TRANSCONDUCTOR EMPLOYING FIELD CONTROLLED -SEMICONDUCTOR Filed oct. 4, 1949 4 sheets-.sheet 2 IN VEN TOR.

Sept. 3o, .1952

o. M. s'rUETzER 2,612,567

TRANSCONDUCTOR EMPLOYING FIELD CONTROLLED SEMICONDUCTOR Filed Oct. 4, 1949 4 Sheets-Sheet 3 j@ ya 7.5

sept. 3o, 1952 o. M. -sTuETzER TRANSCONDUCTOR EMPLOYING FIELD .CONTR-OLLD SEMICONDUCTOR 4 Sheets-Sheet 4 Filed Oct. 4, 1949 E .f ma d N m7 m5 .MF .N ig w .ff

y lPatented Sept. 30, 1952 UNITED STATES 'PATENT OFFICE* Y y I 612567. .y

TRANSCONDUCTOR EMPIIQYING'F'IELD coNrRoLLED snivncoisu)Uo'roRr v Otmar Michael StuetzenDa-yton, Ohio Appliation october 4, 1949, seriall No. 119,541' I Y -2 claims. y(ci. 179-171) y (Granted under the act of March 3, L1883,

'This invention relates to transconductive `de-l vices and particularly to such devices employing semi-conductors as vthe current controlling element. It isthe object of the invention to provide (a device which has .the Vsubstantially unilateralv transductancev or mutual conductance, andl the high input impedance of a three electrode vacuum tube but Whichdoes not require a thermionic -emitter or yan evacuated envelope.

A known type of transconductoremploying'al Semiconductor is described in. an article entitled amendedApril .30, 1928; 370 Of (7757) -vice is extremely high as in the case of a vacuum v tuber` `Thejsignal powerabsorbed by the input circuit therefore approaches zero, so that the power over that of the ftransistor since the high impedance 4between contro-1 electrode and crystal prevents current drain on the bias source and the power-wastage caused thereby.`

. It is therefore a further object of the invention l to provide a transconductor of the `semiconductor The Transistor-A crystal Triode appearing in l theSeptember 1948 issueof ElectronicsnThe transistor described' in this article employs .a

.germanium crystal and two electrodes making ilpoint contact with the crystal at pointsvery Vclose together. A load. impedance. and a voltage acting in the back direction of -the crystal -a're connected between the *crystalA androne of the. electrodes called the vcollector electrode'. A bias voltage. acting in the forward directionof the crystal and a source of ,signal voltage'are connected in series between the crystal andthe other electrode called the emitter electrode. In this device the electrical contact between the emitter or input electrode and the crystal'results in a very loW input impedance which limits therp'ower `amplification 'about .20-decibels. Also the considerable current in the low resistance input circuit due to the forward acting bias .voltage results in a power wastage which lowers the efciency of the device. i

, The transconductor. inV accordance with the invention-utilizes'a semiconductor crystal having an .output electrode makinga point contact therewithas in the case. of the above described transistor. However the input electrode, insteadof making. contact with v.the crystal, is brought as close tothe point of contact 'of the output electrodefasv is possible without contacting the crystal Y or the output electrode..fThe input electrode con- Variations in the input elec- Ethan known devices ofthis type.

-`The physical effects underlying the current controlling action of an electrical fieldin the neightype which has a higher inputimpedance, a higher power amplification and. a. higher efliciency borhood of a point contactv ona relatively thick .semiconductor are not understood at the present time and therefore no theory of. oper-ation yof the 1948. It. is-found, however, thatI the effect produced-by an electrical eld acting in the neighrborhood of a'point contact on a relatively thick semiconductor is from'1000'to 100,000 times greater thancan be'explained by the Shockley-Pearson theory:` -Itf lappears likely, therefore, that' the physical natureof the` effect acting in the de- .scribed transconductor is entirely different from that operating in ythe case of thin films of semiconductorsas described by Shockley and Pearson.

The transconductive device .described may be used-foreX-ample, asa power amplifier, oscillator, rectifier,A frequency converter, phase or polarity inverter, impedance changer, and, ingeneral, may be put'to almost any use, withinits powerhandling limits/,Ito which a vacuum tube may be put. Initspresent stage of development, however, the ldevice is not suitable fas avo-ltage amplifier as the voltage4 gainin` 'most cases will not exceed unity. l 'The inventionwill be 'described in more detail 1 inconnection With several preferred embodiments y thereof as shown in the accompanying drawings,

' Fig. l sho-ws the arrangementof electrodes and semicondi'ictor in a transconductive Vdevice in accordance with the invention; 'l

'Figfla shows a modificationjof Fig, l employ- Aing al dielectric between control electrodes and semiconductor;

Figs. 2, 3 and 4 are characteristic curves o f a transconductive device in accordance with the invention;

Fig. 5 shows the physical arrangement of elements in a practical embodiment of the invention;

Fig. 6 is. a sectional. view of Fig,;5 showing th arrangement of electrodes and spacer;

Fig. 7 is a sectional View of Fig. 6 showing further the arrangement of electrodes and spacer;

Fig. 8 shows an additional electrode, arrangefl ment;' 4 n Fig. 9 shows a circuit employing the electrode arrangement of Fig. 8 to obtain paralleloperation,

of several point contacts on a single crystal;

Fig. 10 shows apparatus for obtaining series oneration of several point contacts;

Fig. 11 shows an arrangement; for usingy the transconductor as a frequency converter; 'and' Fig. 12 shows the transconductive device used as an impedance changer.V

Referring to Fig.l l, asimplined drawing of a transconductor in accordancewith the invention is show-nfV A spacer I-made'of glass orf/other suitableinsulating materialserves to support output electrode IVI and input or'controielectrodes I2 and; I3 in' very close physicall relation-u The output electrode II is broughtinto contact 'with the semiconductory I4, however, the input orcontrol electrodes IZandi'l:v do not touch'the semiconductor but arebrough-t as close tlfieretov as 'possible without makingelectrical contact. The

sizes oithe' electrodesand the spacings are quite small so 'that vfthe controlfelectrodes I2 and`l3V may be positioned as closeas possible' to the-point of contact or the-output'electrode II. Thisenables th'evcontrolfelectrodes to apply and control 'an electrical eldlvin thev immediate neighborlhood. of the point contacti` To thisend the electrodes: II'-, I'Zj 'andI-may? have diameters of from 2l'to 50 microns and the. distance from the center of` electrodeII tothe outermost 'part of electrode `I2 or ,I3-may beg-of the order of 2001 microns. Suitable materials Ifor the electrodes areline lwires.v of tungsten; Vmolybdenum 0r platinum.- The semiconductor Illiis preferably n-type 'Orp-type germanium, p-type4v silicon or tellurium The n and @designation isin accordance with the pres- Vent theory .of conduction within a semiconductor havinga ,slight amount of another element con- --tainediitherein as: an; impurity. Inthe ntylpe i semioonductorfthere are :extra electrons in the interatomi-cbonds between theatomsof the-fprincipal; material ofithe semiconductor and the atoms of; the impurity .a-ndconduction through .this type consists in movement of the excess electrons. lIn vthefiletypesemiconductor -there'is a lack .of `electrcms or .-holes in the interatomic jhondsgand the presence of'xsuch; fholesl makes eletron movement ornzconduction with-in the semiconductor possible. Cuprous Vvoxide and selenium are falso sui-table semiconductors for low v frequencies but; lexhibit Arelaxation effects. which maire them undesirable-for high frequency use. Although only` two input or control electrodes l .Iv-2; and Amarel shown in Figl for-the sake-ofsimrplicity... av practical embodiment of the device should preferably have a ring of ve or'fsixfsuch electrodesuniformly spaced and eXtend-ingzcompletely around the output electrode II. [The inputor control electrodes are connected together and' the. input circuit, containing terminals I5 for the application of. ,a signal `voltage and bias voltage source I6,r extendsfrom theparallel connected.. control electrodes to the-.semiconductor I4. The output circuit is connected between 4 the output electrode I I and semiconductor I4 and contains load impedance II and a source of direct current I8. v

For maximum power output the voltage Vb of source I8 should act in the back direction, or

ydirectioniof` greater resistance of theisemiconductor. For n-type germanium the back direction is with the contacting electrode negative and the semiconductor positive. With the crystal operated. in. its back direction the output impedance of. the device may be from 5'000 to 100,000

ohms depending uponthe semiconductor used, contact area, contact pressure, etc. The device also may .be operated with the voltage acting in the forward direction of the semiconductor and such operation has the advantage of lower noise generation and smaller relaxation effects; however,'forA suolioperation, the voltage Vg should be keptv below the contact potential of the contacting electrode and the semiconductor or else propercarelv for matching the low outputzimnamely,l T5' rnicroam-peres@ llllmicrcamperes.

The' mutual conductance olf'ithel device. at any particularl value'V ot Vr'would-.be-equalf to the derivative-ofi the curve atfthat point;.i' It'will be 'notedr that. the Ym-utiflalconducta'nce-1, 1.2 e; thedevriva-tive. oif the curves.. passesA through zero. and

lesponds-'veryl closelywto the negative potential on thea-surface of the semiconductor .due toi the-'con- .tactfcurrent,. or, in. other words, .the potential ditiference and :resulting 1 'field between .the control electrode and the` semiconductor zero. .atfthis 'Figfa shows the. static :outputfelectro de. voltageoutput lelectrode 'current:characteristics Withbias voltage; V'g as. a. parameter. 'The' characteristics are `for-a device operated in the-negative slope orH negativerv` mutual conductance. region.

' Figi, 4. gives. curves of mutual conductance Gm,

output'electrod'c cur-rentrls, and. Idynamic. output electrode. resistance. Ra, plotte'd againstw back voltage Va for avalueL oiVg.=0. This ligure/also `shows the effec-.t of illumination of the vpoint-contact on-Gm-and le. It is seen thatthe dynamic output electrode resi'stanceRa=dVt/di lies within thev limits of approximately 25,000l ohms1 .and 65.0.00 ohms.. It. is alsov seen that.v the mutual conductance Gm, is. approximately proportional Vto outputelectrode current;

lnapparatus constructed in accordancewrifth Fig,` l'in which the control electrodesvdo not touchthe semiconductor or the output" electrode the:y input 'impedance is theoretically infinite. In prac-.tice ofcourse. the attainment of thi-sideal' is ifrnpossibleand; under thev 4best. conditions. input impedances; of i000 to r10,000 megohms will'be measured'. Furthermore, it has. been found.' lexpermental'ly .that the; controlling. elect4 of ythe input electrodescan be :increased byllingzthe space: betweencthe-'electrodes and; the vsemiconductor with aarnaterial of high dielectricycon- .-stagnt. such as titaniumdioxide 0r-V titaniuntborate powder, bariumtitanate,'trlethylene glycoll, glycvis not dicult.

-crine, oil, Yamorphous carbon or distilled water as shown in' Fig. la.. AThese materials mayi'be pressed with a comparatively high force betweeny the control 'electrodesy and the semiconductor. Another' method of obtaining ex-tremelyclose spacing with dielectric separation is to femploy' control electrodes of aluminum, first oxidized onv the ends to form an insulating surface layer and e conductance is increased by a factor roughly equal to the dielectric constant of the insulating material used. Typical devices using adielectric between the control electrodes rand the semiconductor have exhibited voltage ampliiications of approximately unity and current amplications of 100 t0 1000. 'A practical embodiment of a transconductive device in accordance with the invention yisfshown in Fig. -5. The various elements are shown mounted on a standard octal base. A 'crystal'cf usemiconductive material I4 'is mounted' von plunger I9 which slidably ts into cylindrical member 20. Member 2B contains an 'internal coil spring (not shown) which forces the crystal I4 against the end of electrode spacer 23. vScrew 2| and lock nut 22 serve to adjust the pressure exerted by the spring against the crystal. The

whole assembly is attached to pin 24 of the octal base which serves to make electrical lcontact with f the crystal. The spacer 23 is held by an encircling metallic strap 25 which ,is mounted on octal base pin 2l by means of bracket 26.

The spacer 23 may be made of glass or other suitable insulating material such as quartz and contains a centrally located Wire 29 and six wires .30 positioned uniformly around the central wire. The outer wires converge on the vcentral wire until at the end of the spacer all wires are very closely spaced but nevertheless are electrically insulated from each other. The appearancey of the end of the spacer 23 is shown in Fig. 6. Although the separation of the wires at the end of the spacer is so small as to be measured in microns the physical realization of such spacings The processes for producing spacers of this type are to be covered in a separate patent application. Brieiiy, however, one

vprocedure is to start with a glass cylinder of convenient size having the required number of holes therethrough. This cylinder vis then reduced in size by repeated fusing and drawing.

vAs the size becomes small enough to' be fragile a glass tube is closely fitted over vthe cylinder and the drawing continued. The process of drawing and increasing the wall thickness by adding glass tubes is continued until the holes have reached a diameter of three to ve times is then cut at this point and ground-back in the direction of divergency until electrical` separation of the wires is obtained, as indicated, 'for example, by an ohmmeter.

Fig. 7 shows a longitudinal cross-section of spacer 23.` In grinding a flat surface inthe' end off'th'e spacer the electrodes, if made of the harder metals-will protrude slightly above the surface ofr the"y glass as vshown-'in r`the "case Cif-'electrode 29. This extension *serves to'contact the crystal whenthelatter is `pressed against the' spacer.

`Th=;"'electro"des 30, Which'are not supposed to touch lthe crystal, may be-connected to a source of vpotential and etched back inan electrolyte.- 9

Theamount ofr etching and 4hence the'clearance between the control electrodes .and the crystal Vmay* be accurately 'controlled by the voltage,

timefand vconcentration of -theelectrolyte` Referring again to Fig. 5 the' wires3llare clamped under the Lmetallic strip 25-so as to fm'ake an electrical connection between pin 21 andthe control electrodes." Wire 29, which forms the output electrode,l is attached to pinA 3|.l -Several transconductors' 1 ofv the above described type may be operated in parallel on a single semiconductor crystal. One method of accomplishing this is to form a spacer having two rings of electrodes around a center electrode Aas shown in Fig. 8. The center electrode and outer ring of electrodes may act as control electrodes with the inner ring of electrodes acting as output electrodes. The circuit connections are as shown that of the wires to be inserted. 'The wires are then inserted into the passageways, and the drawing continued until the desired spacing is obtained. Another procedure is to start with a bundle of glass tubes of convenient size with alarger glass tube fitted over the outside of the bundle. This assembly is then fused anddrawn down to the required size, adding additional tubes on the outside to build up the wall thickness as in the preceding case. The last drawing should be made adjacent to a cold section of the cylinder so as toresult in a conical shape as shownpin Fig. 5. This permits mutual divergence of the wires away from the crystal end in Fig. 9 in which the `dark points indicate electrodes that touch the semiconductor.

Several `transconductors may also be Operated in series as shown in Fig. 10. In this arrangement a plurality'of discs of semiconductive material I4 are separated by particles of metal V32 such as metal balls. The resulting pile of alternate discs and particles is contained in a glass tube 33, the inside diameter of which may be of the order of 1/2 millimeter. 34 is sputtered on the outer surface of the tube to actas la control electrode. The ends ofthe tube may be sealed by plugs 35 and 3B of plastic silver. The input circuit isrconnected between electrode 34 and plug 36 and the 'output circuit between plug 35 and plug 38.

In cases where the transconductor is to be used at high frequencies the interelectrode capacityV between control electrodes and output electrode may become troublesome. rangement such as shown inFi'g. 8 may be used making the center electrode the output electrode, the outer ring of electrodes, the control electrode and using the intermediate ring of electrodes as Y a screen grid.

Fig.' 11 illustrates `the manner 'of using the transconductive deviceasa frequency converter. In the example shown an electrode structure such as shown in Fig. 6 is used with one of the fref quencies being applied to half the control electrodes connected in parallel and the other fre- A metallic layer p In this case an ar- The center electrode emacs# ling the bjias voltage from -40fvolts to- +-1-0- volts.

The. possibility of using the device for Yvolume control by varying the biasing voltage willvalso be apparent from the curves.

The transconductve device can also be advantageously` used-,incombination with -a transistor,,descri-bedfearlier in the specification, the device working as a preamplier and atthefsame time asvan impedance matching device between a high impedance signal source and: the low input impedance of. the transstor Forthispurpose both devices may be .arranged on the same semi.- conductor as shown in Figi. 12 in which l,3l 4and 3% represent the'collector andemitterelectrodes, respectively, of the transistor and 39 and 40- representthe outpu-t andinput*electrodes,.respec tively, of a transconducti-vezdevice in` accordance with the invention. y Y y I- claim:

Y1. AA transconductive -device comprising a thin walled tubeof 'insulating material,` a-pile in said tube consisting of alternate metal particles and discs of sem-iconductive material, each ymetal -particle being in electrical. contact-with each disc 8 adjacent thereto whereby said particles and discs are inseries conductive relationship, means for making electrical connections to the ends vof said pile; and a control electrode consisting of-"a metallic layer on the outside .of said tubean'd coextensive with said pile.

, 2. lApparatus as claimed in claim 1 inwhich an output circuit comprising a load impedance and a source of direct potential is connected between the .ends of said: pil'e and in which a source ofsignal and a biasing, meansV is connected between said control electrode and one end. of said pile.. OTMAR MICHAEL STUETZER;

REFERENCES CITED The following references are of record in the .File of this patent:

UNITED .STATES PATENTS l2,540,490 .Rittnel' Feb; 6,1951 

